Fix non-portable path serialization in ProjectNode.

This was failing snapshot tests on the Linux CI machines,
since I committed snapshots with backslashes.

I think the old path serializer was still the wrong approach,
this one is sort of a middleground but I'm still not super
happy with it.

src/snapshots/serve_session__change_file_in_project_after.snap

@@ -29,7 +29,7 @@ children:
             children: {}
             properties: {}
             ignore_unknown_instances: ~
-            path: "/foo\\file.txt"
+            path: /foo/file.txt
       relevant_paths:
         - /foo/file.txt
         - /foo/file.meta.json

src/snapshots/serve_session__change_file_in_project_before.snap

@@ -29,7 +29,7 @@ children:
             children: {}
             properties: {}
             ignore_unknown_instances: ~
-            path: "/foo\\file.txt"
+            path: /foo/file.txt
       relevant_paths:
         - /foo/file.txt
         - /foo/file.meta.json